Add the chicken thighs to the slow cooker. Top with the diced onion, minced garlic, lemon zest, lemon juice, chicken broth, oregano, olives, salt, and pepper.
Cover and cook on low for 6 hours until the chicken is very tender.
Remove the chicken from the slow cooker and shred it using two forks. Return the shredded chicken to the pot.
Stir in the orzo until it is fully mixed into the broth.
Cover and cook on high for 20 to 25 minutes until the orzo is tender.
Divide into bowls and top each serving with crumbled feta and chopped parsley.